Effect of growth hormone on rat hepatic cytochrome P-450f mRNA: a new mode of regulation

Endocrine disrupting chemicals and metabolic disorders in the liver: What if we also looked at the female side?

Endocrine disrupting chemicals (EDCs) are linked to the worldwide epidemic incidence of metabolic disorders and fatty liver diseases, which affects quality of life and represents a high economic cost to society. Energy homeostasis exhibits strong sexual dimorphic traits, and metabolic organs respond to EDCs depending on sex, such as the liver, which orchestrates both drug elimination and glucose and lipid metabolism. In addition, fatty liver diseases show a strong sexual bias, which in part could also originate from sex differences observed in gut microbiota. The aim of this review is to highlight significant differences in endocrine and metabolic aspects of the liver, between males and females throughout development and into adulthood. It is also to illustrate how the male and female liver differently cope with exposure to various EDCs such as bisphenols, phthalates and persistent organic chemicals in order to draw attention to the need to include both sexes in experimental studies. Interesting data come from analyses of the composition and diversity of the gut microbiota in males exposed to the mentioned EDCs showing significant correlations with hepatic lipid accumulation and metabolic disorders but information on females is lacking or incomplete. As industrialization increases, the list of anthropogenic chemicals to which humans will be exposed will also likely increase. In addition to strengthening existing regulations, encouraging populations to protect themselves and promoting the substitution of harmful chemicals with safe products, innovative strategies based on sex differences in the gut microbiota and in the gut-liver axis could be optimistic outlook.

A novel vascular EET synthase: role of CYP2C7

We demonstrated previously that cytochrome P-450 (CYP) 2C29 is the epoxyeicosatrienoic acid (EET) synthase responsible for the EET-mediated flow/shear stress-induced dilation of vessels of female nitric oxide (NO)-deficient mice (Sun D, Yang YM, Jiang H, Wu H, Ojami C, Kaley G, Huang A. Am J Physiol Regul Integr Comp Physiol 298: R862-R869, 2010). In the present study, we aimed to identify which specific CYP isoform(s) is the source of the synthesis and release of EETs in response to stimulation by shear stress in vessels of rats. Cannulated mesenteric arteries isolated from both sexes of N(G)-nitro-L-arginine methyl ester (L-NAME)-treated rats were perfused with 2 and 10 dyn/cm(2) shear stress, followed by collection of the perfusate to determine EET concentrations and isoforms. Shear stress stimulated release of EETs in the perfusate of female (but not male) NO-deficient vessels, associated with an EET-mediated vasodilation, in which 11,12- and 14,15-EET contributed predominantly to the responses. Rat CYP cDNA array screened a total of 32 CYP genes of mesenteric arteries, indicating a significant upregulation of CYP2C7 in female L-NAME-treated rats. Endothelial RNA and protein were extracted from intact single vessels. Expression of CYP2C7 mRNA and protein in pooled extractions of endothelial lysate was identified by PCR and Western blot analyses. Transfection of the vessels with CYP2C7 short interfering RNA eliminated the release of EETs, consequently abolishing the EET-mediated flow-induced dilation; these responses, however, were maintained in vessels transfected with nonsilencing short interfering RNA. Knockdown of endothelial CYP2C7 was confirmed by PCR and Western blot analyses. In conclusion, CYP2C7 is an endothelial EET synthase in the female rat vasculature, by which, in NO deficiency, shear stress stimulates the release of EETs to initiate vasodilation.

Growth hormone-regulated periportal expression of CYP2C7 in rat liver

Most drug- and steroid-metabolizing cytochrome P450 (CYP) enzymes are expressed in the mammalian liver in a characteristic zonated pattern, with high expression in the downstream perivenous (centrilobular) region. Here, we report that CYP2C7, a member of the rat CYP2 family, is expressed preferentially in the opposite, periportal region. CYP2C7 mRNA, as detected by reverse transcription-polymerase chain reaction, was detected almost exclusively in cell lysates obtained from the periportal region, indicating a very steep acinar gradient. The amount of immunoreactive CYP2C7 protein in periportal cell lysates was also higher than in samples from the perivenous region. This gradient was reversed by hypophysectomy, which markedly and selectively reduced the periportal CYP2C7 protein content. Subsequent growth hormone infusion by osmotic minipumps restored the zonation by selectively increasing the amount of periportal CYP2C7 protein. Although hypophysectomy suppressed CYP2C7 mRNA and growth hormone counteracted it, regulation at this level did not appear to occur in a zone-specific fashion. This indicates that growth hormone-mediated zonal regulation of CYP2C7 protein has additional translational or posttranslational components. Ethanol treatment, which has been shown to affect growth hormone levels, significantly induced CYP2C7 mRNA, but not zone specifically. Our results demonstrate that growth hormone up-regulates the CYP2C7 gene by enhancing the expression of the protein specifically in the periportal liver region. Growth hormone may up-regulate other periportally expressed liver genes in a similar fashion.

Zonation of hepatic cytochrome P-450 expression and regulation

The CYP genes encode enzymes of the cytochrome P-450 superfamily. Cytochrome P-450 (CYP) enzymes are expressed mainly in the liver and are active in mono-oxygenation and hydroxylation of various xenobiotics, including drugs and alcohols, as well as that of endogenous compounds such as steroids, bile acids, prostaglandins, leukotrienes and biogenic amines. In the liver the CYP enzymes are constitutively expressed and commonly also induced by chemicals in a characteristic zonated pattern with high expression prevailing in the downstream perivenous region. In the present review we summarize recent studies, mainly based on rat liver, on the factors regulating this position-dependent expression and induction. Pituitary-dependent signals mediated by growth hormone and thyroid hormone seem to selectively down-regulate the upstream periportal expression of certain CYP forms. It is at present unknown to what extent other hormones that also affect total hepatic CYP activities, i.e. insulin, glucagon, glucocorticoids and gonadal hormones, act zone-specifically. The expression and induction of CYP enzymes in the perivenous region probably have important toxicological implications, since many CYP-activated chemicals cause cell injury primarily in this region of the liver.

CYP2C7 expression in rat liver and hepatocytes: regulation by retinoids

Rats deficient in vitamin A express low levels of P4502C7 mRNA in the liver. Administration of all-trans retinoic acid (at-RA) or growth hormone (GH) to deficient animals only partially restored the expression whereas the combined treatment returned the P4502C7 mRNA levels to that observed in normal rats. That a retinoid is the predominant inducer of P4502C7 at the cellular level is evident from studies performed with primary hepatocytes, but it became clear that GH is a prerequisite for the vitamin A effect in vivo. The at-RA induction of P4502C7 mRNA in primary rat hepatocytes was inhibited by ketoconazole, an inhibitor of P450 activity, and by cycloheximide, blocking ongoing protein synthesis. In contrast, the at-RA induction of RAR-beta2 mRNA was not affected by any of these compounds. This could indicate previously not recognized mechanisms of at-RA action. Interestingly, at-4-oxo-RA, an at-RA metabolite formed by a P450 catalyzed reaction, also induced P4502C7 mRNA. Induction of P4502C7 mRNA by the retinoic acid receptor (RAR) selective agonist TTNPB indicated that this pathway is preferred over the retinoid X receptor (RXR) pathway. In addition, analysis of RA metabolites in liver cell extracts revealed the formation of several as yet unidentified metabolites. The formation of some of these metabolites was inhibited by ketoconazole and they could therefore constitute potential inducers of CYP2C7. We suggest that metabolism of at-RA, possibly by a P450 enzyme, is an important step in the at-RA induction of P4502C7.

Spi-1: an hepatic serine protease inhibitor regulated by GH and other hormones

A sensitive RNAse protection method was used to show that serine protease inhibitor-1 (Spi-1) is expressed in rat liver and heart, but not in kidney or brain. Bovine somatotropin (bGH) and placental lactogen (bPL) induced rat hepatocyte cultures to express both Spi-1 and IGF-1 mRNA, with bPL approximately 100-fold more potent than bGH. Bovine prolactin (bPrL) did not induce hepatocyte Spi-1 mRNA, demonstrating lack of involvement of lactogenic receptors. Albumin mRNA levels were stable during hepatocyte culturing and were unaffected by growth hormone (GH) treatment, showing that neither culture conditions nor GH treatment affected cellular differentiation. Eliminating serum-free medium hormone supplements one at a time, estradiol, testosterone and T3 were shown to be unnecessary for GH induction of Spi-1, while dexamethasone removal decreased Spi-1 mRNA levels to 10% of GH-stimulated controls. bGH induction of Spi-1 mRNA in the presence of only dexamethasone and glucagon was 75% higher (p < 0.01) than levels seen with insulin also present.

Regulation of liver-specific steroid metabolizing cytochromes P450: cholesterol 7α-hydroxylase, bile acid 6β-hydroxylase, and growth hormone-responsive steroid hormone hydroxylases

The hydroxylation of cholesterol, bile acids, and steroid hormones by liver cytochrome P450 (CYP) enzymes proceeds with a high degree of regiospecificity, and contributes to both biosynthetic and catabolic pathways of sterol metabolism. CYP 7-catalyzed cholesterol 7α-hydroxylation, a key control point of bile acid biosynthesis, is regulated at a pretranslational step, probably transcription initiation, by multiple factors, including liver bile acid and cholesterol levels, thyroid hormone status, and diurnal rhythm. Hydrophobic bile acids, such as lithocholic acid, are converted to less cholestatic derivatives by 6β-hydroxylation carried out by CYP 3A P450s, which also catalyze steroid hormone 6β-hydroxylation reactions. Complex, gender-dependent developmental patterns characterize the expression of steroid 5α-reductase and several rat liver steroid hydroxylase CYPs. Multiple pituitary-dependent factors regulate the expression of these enzymes; of greatest importance are the gonadal steroids and the sex-dependent secretory patterns of growth hormone (GH) that they impart. The continuous presence of GH in circulation, a characteristic of adult female rats, positively regulates expression of the female-specific steroid disulfate 15β-hydroxylase CYP 2C12, while expression of the male-specific steroid 16α- and 2α-hydroxylase CYP 2C11 is stimulated by the intermittent pituitary secretion of GH that occurs in adult male rats. Intermittent GH can stimulate CYP 2C11 gene expression even when the hormone presents to the hepatocyte at a non-physiological pulse amplitude, duration, and frequency, provided that an interpulse interval of no GH (obligatory recovery period) is maintained for at least 2.5 h. GH regulates the expression of the CYP 2C11 and CYP 2C12 genes at the level of transcription initiation. This process is probably mediated by sex-dependent and GH-regulated protein-DNA interactions, such as those observed in the 5'-flank of the CYP 2C12 gene. Thyroid hormone is a second major regulator of liver steroid hydroxylase P450 activity. It regulates these enzymes directly, at a pretranslational step, and indirectly, through its stimulation of pituitary GH secretion and by its positive effects on the expression of the flavoenzyme NADPH-P450 reductase, which catalyzes electron transfer that is obligatory for all microsomal steroid hydroxylation reactions.

The role and mechanism of growth hormone in the regulation of sexually dimorphic P450 enzymes in rat liver

The determination of sexually dimorphic hepatic steroid metabolism in rat liver has been shown to involve growth hormone. However, the mechanisms by which growth hormone controls the cytochrome P450 enzymes responsible for this dimorphic steroid metabolism is largely unknown. In this review we discuss different levels of growth hormone signal transduction, including receptor binding, signal transduction and activation of target genes by growth hormone.

Effects of testosterone and estrogen on hepatic levels of cytochromes P450 2C7 and P450 2C11 in the rat

The effects of exogenous hormone treatment on the expression of cytochromes P450 2C7 and P450 2C11 were studied in neonatally gonadectomized and sham-operated male and female rats. Hepatic levels of cytochrome P450 2C7 were found to be two- to threefold higher in intact adult female versus male rats. Neonatal gonadectomy resulted in a reversal of the relative cytochrome P450 2C7 levels in male and female animals at maturity. Expression of this isozyme was restored in ovariectomized females by estradiol treatment. Furthermore, neonatal and/or pubertal administration of estradiol to intact male rats induced cytochrome P450 2C7 to adult female levels. On the other hand, administration of testosterone at all times examined had no effect in intact female rats, but decreased cytochrome P450 2C7 to normal levels in neonatally castrated males treated during adulthood. Neonatal testosterone treatment also increased hepatic cytochrome P450 2C7 content in both ovariectomized females and intact males. These results indicate that estrogen is required for full expression of cytochrome P450 2C7 while the effect of testosterone is ambiguous. In comparison, neonatal gonadectomy of male rats abolished the adult expression of cytochrome P450 2C11. Normal levels were restored only by treatment with testosterone during adulthood. Neonatal testosterone treatment did not induce cytochrome P450 2C11 levels in gonadectomized rats of either sex. In contrast, neonatal estrogen treatment suppressed cytochrome P450 2C11 expression in intact adult male rats to the same extent as neonatal castration. These results indicate that androgen exposure during the adult, and not the neonatal, phase is essential for full expression of cytochrome P450 2C11.

Sexual differentiation and regulation of cytochrome P-450 CYP2C7

The multigene family of proteins known as the cytochrome P-450-dependent monooxygenases play a central role in the metabolism of hormones and foreign compounds. As part of our studies into the function and regulation of these proteins we have isolated a little studied constitutively expressed isozyme CYP2C7 and have investigated its substrate specificity and mode of regulation. Interestingly the haem of this enzyme in its isolated form is almost 100% in the high spin state. The enzyme was active in the metabolism of a range of model resorufin substrates, but exhibits highest activity towards benzyloxyresorufin. Indeed, this isozyme appears to play a significant role in the metabolism of this substrate in microsomal samples from untreated male rats. Tissue distribution studies indicated that CYP2C7 was expressed in liver, kidney and possibly muscle tissue. Cytochrome P-450 CYP2C7 could not be significantly induced by any of a wide range of known modulators of cytochrome P-450 expression at the mRNA level, however some significant changes in protein expression were observed. Some of the agents used (e.g., diethylnitrosamine and carbon tetrachloride) caused a significant reduction in the expression of this protein. In agreement with other reports where mRNA levels were measured we found that the level of CYP2C7 protein expression was sexually differentiated. Female rats express two to three times the level found in males, the sex difference being reversible by hypophysectomy.

Regulations of male-dominant P-450Md mRNA in rat liver by hormonal factors and xenobiotics

Male-dominant P-450Md mRNA was undetectable in the livers of newborn rats. In female rat livers, the mRNA appeared at 17 days of age and then decreased to very low levels in the adult periods. The level of P-450Md mRNA in female rats was increased by phenobarbital or dexamethasone treatment, whereas the level in the males was depressed by methylcholanthrene. Hypophysectomy decreased the level of P-450Md mRNA in male rat livers, and continuous infusion or twice-daily injections of growth hormone to hypophysectomized rats caused further suppression or clear restoration, respectively, of the mRNA level.